The amount of voice memo recorded will depend entirely on the amount of space available on your iPhone + the state of battery power left. It all depends on how long your fully charged battery could last. It is for you to test.

I am not sure but the file-size could be limited to 2GB (which is pretty much huge for a voice memo). You roughly record 2 MB per minute of voice memo. This means that you have some 25+ hours of recording for a single voice memo, of course, if your iPhone available space and battery limitations allow it.